The Marvel Universe was envisioned in the early 1960s by Marvel founder Stan Lee, alongside original artists Jack Kirby and Steve Ditko. As Lee envisioned it, the Marvel Universe would be the place where all of their creations could exist in one singular fictional reality. That way, Spider-Man could meet up with the Fantastic Four to take on Galactus together, and the reader would know it was all happening in the same world.
The Marvel Universe is where Captain America lived (and died, and lived again) and it's the place Spider-Man lived (and died, and lived again). It's where The X-Men, The Avengers, Daredevil, and The Punisher all reside. It's the realm of the Inhumans, Shi'ar Empire, Guardians of the Galaxy, and Silver Surfer. It is not, however, where Batman, Wonder Woman, Superman, or The Green Lantern live. (That is the DC Universe, and it's a whole different thing entirely). Think of the Marvel Universe as our own universe, only with a host of super-powered beings, mutants, gods, magic, and alien life-forms constantly battling it out.